Maria Sharapova was guilty of "willful negligence" for using meldonium, and international tennis officials were aware that many players were taking the drug before it was banned this year, former World Anti-Doping Agency president Dick Pound said Wednesday.

Pound told The Associated Press that Sharapova could face a ban of up to four years unless she can prove mitigating circumstances to explain her positive test for meldonium at the Australian Open in January.

Meldonium, a Latvian-manufactured drug designed to treat heart conditions, was added to the World Anti-Doping Agency's banned list on Jan. 1 after authorities noticed widespread use of the substance among athletes.

In announcing her positive test at a news conference in Los Angeles on Monday, Sharapova said she had been using the drug for 10 years for various medical issues. The five-time Grand Slam champion and world's highest-earning female athlete said she hadn't realized meldonium had been prohibited this year, taking full responsibility for her mistake.

"An athlete at that level has to know that there will be tests, has to know that whatever she or he is taking is not on the list, and it was willful negligence to miss that," Pound said. "She was warned in advance I gather. The WADA publication is out there. She didn't pay any attention to it. The tennis association issued several warnings, none of which she apparently read."